With the higher speed and the higher density of electronic devices in recent years, EMI (Electro Magnetic Interference) from the electronic devices only been increased. Because EMI radiated from an electronic device influences other electronic devices, EMI is restricted by various standards and, to meet these standards, a spread spectrum clock technique that effectively reduces EMI noises is getting more important.
For an image reading apparatus applicable to a scanner, a digital copying machine, a digital color copying machine, a facsimile machine, etc., known techniques concerning EMI countermeasures thereof can be those disclosed in Japanese Laid-Open Patent Publication Nos. 2001-94734 and 2001-268325. According to these techniques, a CCD is used that photo-electrically converts a reflected light beam from a script to be read into image data, a source oscillation clock is used as a signal to control an analog front end (AFE) that adjusts the offset and the gain of the image data, and image processing units other than the above use a spread spectrum clock that is frequency-spread.
More specifically, an image reading apparatus disclosed in Japanese Laid-Open Patent Publication No. 2001-94734 is adapted to divide a timing circuit of the image reading apparatus into an analog signal processing system clock generating circuit and a digital signal processing system clock generating circuit, to use in the analog signal processing system clock generating circuit a reference clock from a reference clock generator, and to use in the digital signal processing system clock generating circuit a spread spectrum clock from a spread spectrum clock generator.
An image reading apparatus disclosed in Japanese Laid-Open Patent Publication No. 2001-268325 is adapted to divide a timing signal generating unit of the image reading apparatus into an analogue signal processing system clock generating unit and a digital signal processing system clock generating unit, to be able to select a modulation width optimal for each system, that is, a modulation width by differing the modulation width of a spread spectrum clock generated by a spread spectrum clock generating unit and, therefore, to be able to suppress generation of image noises, and to be able to obtain an EMI reduction effect.
In a scanner unit of an image reading apparatus, a linear image sensor (CCD) and an analog front end (AFE) that handle an analog signal need to be operated with a clock at a specific frequency. However, a clock at a specific frequency is disadvantageous against the regulations concerning the EMI countermeasures and, therefore, it is general to use a spread spectrum clock for a digital signal processing section.
Similarly, in each of the image reading apparatuses disclosed in Japanese Laid-Open Patent Publication Nos. 2001-94734 and 2001-268325, a CCD and an analog front end that handle an analog signal are operated with a clock at a specific frequency and an image processing section that handles a digital signal is operated with a spread spectrum clock.
However, although the frequency of the clock at a specific frequency and the frequency of the spread spectrum clock are equal when they are averaged, the timing in each cycle is not necessarily same and, therefore, in synchronized transferring, difference is generated between the term during which a pre-stage of image processing holds data and a timing at which a post-stage of the image processing captures the data. An image processing circuit is designed taking a margin such that data can be accurately delivered even when difference is generated between the timings. However, the designing becomes difficult as further higher performance concerning the resolution, the gray scale, and the reading speed is demanded and the transferring speed becomes higher. Therefore, it is difficult to secure sufficiently the timing for latching an image signal that is inputted from the analog front end (AFE) into an image processing IC including an ASIC, etc., in the post-stage.